In the field of microelectronic materials processing, there is a need to selectively deposit and etch metal films, glass and layers of other materials such as polymers. Projection etching using lasers directed through non-contact masks onto the targets have been developed in response to the need to deposit and etch efficiently without excessive exposure of the target to photolithographic contact masks, photoresists and/or patterned transfer layers, and the various processing and cleaning solutions involved in standard contact mask processing. In addition, the distance between the mask/optical system and the target reduces the potential for contamination of the mask/optical system by the resultant etch products.
As disclosed in several patents of Chen, et al, specifically U.S. Pat. Nos. 4,490,210, 4,490,211 and 4,478,677, all assigned to the present assignee and incorporated by reference herein, the laser energy may be directed through a mask which is adjacent to the target material such that it falls directly onto the target, in much the same way as the photolithographic lift-off masks which are well known in the art; or, the mask can be situated between the laser and a projector arrangement wherein the patterned laser image emerging from the mask is processed by the projector (for example: reduced) and directed onto the target. The target substrate may be mounted in air or, more frequently, is mounted in a chamber which is to be evacuated and subsequently filled with an ambient. The ambient is selected to react with the material on the surface of the target to create a product which will volatize when irradiated by the laser energy. Since the laser beam has been patterned, only that ambient-surface material compound in the path of the transparent areas of the mask, and consequently in the path of the laser energy, will be volatized and therefore etched. Many materials can be etched "directly" by the laser energy such that they will volatize in air or in a vacuum without the need for an intermediate step of creating a volatizable ambient-material compound. The lasers used are ordinarily high power and high energy, with an energy density, or fluence, in the range of several hundred millijoules per centimeter squared.
The Chen, et al patents describe the non-contact masks used as being " . . . of a conventional type having a transparent body upon which an opaque pattern is formed[.] [t]he opaque pattern corresponding to the areas of the substrate not to be etched." (See U.S. Pat. No. 4,478,677 at Column 9, lines 18-21 and U.S. Pat. No. 4,490,210 at Column 5, lines 53-56). The mask material is further discussed in U.S. Pat. No. 4,490,211 as having a transparent substrate/body of UV grade quartz with a patterned chromium film thereon. It has been found, however, that the standard chromium masks cannot withstand laser fluences of the order encountered when working with excimer or other lasers having the necessary intensity to etch the target materials. Although opaque to UV light and fairly reflective at visible wavelengths, in addition to being durable and easy to pattern, chromium absorbs approximately 35% of the light at 308 nm and 46% at 248 nm. Therefore, a single excimer laser pulse can easily ablate the chromium and destroy the pattern. Discussions of laser-induced damage can be found in the proceedings of the annual conference on laser damage, "Laser Induced Damage in Optical Materials", 1979-NBS Special Publication 568, Library of Congress Catalog Card No. 80-600110.(Subsequent conference proceedings can also be located at that repository.)
It is therefore an objective of the present invention to provide a mask material suitable for use with high energy, high power lasers in projection etching arrangements.
It is another objective of the present invention to provide a method for making such suitable masks.
It is a further objective of the invention to fabricate a mask having areas transparent to the laser to allow passage without significant loss in intensity and opaque areas capable of reflecting the laser energy without degrading the reflective material in those opaque areas.